Sense of smell is our fastest warning system.

Researchers at Karolinska Institute have found that human avoidance reactions to adverse smells associated with risk are unconscious and very fast. The olfactory organs account for about 5% of the human brain, allowing us to distinguish millions of different smells. Most of these odors are associated with threats to our health and survival, such as chemicals and rotten food. The odor signal reaches the brain within 100 to 150 milliseconds of inhalation through the nose.

It has long been a mystery for neural mechanisms to engage in converting unpleasant smells of humans into avoidance behavior. One reason is that there is no non-invasive way to measure the signal of the olfactory bulb, the first part of the brain that helps detect and remember threatening and dangerous situations and substances.

Researchers at Karolinska Institute developed a way to measure signals from human olfactory bulbs for the first time, which can process odors and transmit signals back to the brain that controls movement and behavior.

The findings are based on three experiments in which participants were asked to evaluate their experiences with six different smells while the electrophysiological activity of olfactory bulbs was measured in response to each odor.

Johan Lundström, the last author of the study, associate professor at the Department of Clinical Neuroscience, Karolinska Institute, said: “It was clear that the bulb reacts specifically and rapidly to negative smells and sends a direct signal to the motor cortex within about 300 ms.” He added, “The signal causes the person to unconsciously lean back and away from the source of the smell. The results suggest that our sense of smell is important to our ability to detect dangers in our vicinity, and much of this ability is more unconscious than our response to danger mediated by our senses of vision and hearing.”

The study was funded by the Knut and Alice Wallenberg Foundation, the National Institute on Deafness and Other Communication Disorders and the Swedish Research Council. There are no reported conflicts of interest.

Story Source & Image Credit : Materials published by Karolinska Institute,

Journal Reference: Behzad Iravani, Martin Schaefer, Donald A. Wilson, Artin Arshamian, Johan N. Lundström. The human olfactory bulb processes odor valence representation and cues motor avoidance behavior. Proceedings of the National Academy of Sciences, 2021; 118 (42): e2101209118 DOI: 10.1073/pnas.2101209118

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